The present invention relates generally to semiconductor lasers, and, more particularly, to devices in which one or more semiconductor lasers are integrated with electronic devices on a single substrate. Integration of optical and electronic components would ultimately reduce the manufacturing costs of devices such as laser light sources, detectors and repeaters for fiber optic communication systems. It is current practice to manufacture laser and electronic components as separate devices. The reason for this is that, although a laser and an electronic device may both be fabricated in semiconductor form, they have physical requirements that are incompatible in one important respect.
A semiconductor laser should ideally be fabricated on a substrate of relatively low resistivity, presenting a relatively low series resistance to the flow of current through the device. However, a low-resistivity substrate is contrary to the usual requirements of a field-effect transistor (FET), which is the transistor type best suited for integration with lasers on a large scale. The other major transistor type is the bipolar transistor, which poses significant electrical isolation problems when multiple devices are incorporated onto one substrate. U.S. Pat. No. 4,092,614 issued to Sakuma et al. discloses a semiconductor device including both a laser and bipolar transistor circuitry. However, the laser is not really integrated into the device. Instead it is mounted as a discrete component on a copper metalization layer over the transistor components.
Field-effect transistors, and in particular those of the same general type as the metal-insulator-semiconductor field-effect transistor (MISFET), do not suffer from the isolation problems of bipolar transistors, and lend themselves well to integration on a large scale. These devices use a relatively shallow surface layer for conduction of current. This is referred to as the active layer or the channel region of the device, or more typically as the n channel or the p channel, depending on the polarity of the current carriers in the channel. Ideally, the material beneath the channel is an insulating or semi-insulating substrate, to confine the current flow to the channel. This semi-insulating substrate is, of course, incompatible with the need for a relatively low-resistivity substrate for a semi-conductor laser.
One solution to this problem is to fabricate the laser as a mesa structure above a semi-insulating substrate, as suggested in U.S. Pat. No. 4,212,020 issued to Yariv et al. However, the height of the mesa interferes significantly with planar fabrication techniques. Masks used in fabrication cannot be as easily aligned on such a structure, and the precision of the manufacturing process is considerably reduced.
It will be appreciated from the foregoing that there is still a significant need for improvement in the area of integrated semiconductor lasers and electronic devices. In particular, the need is for a solution to the difficulties posed by the fabrication of semiconductor lasers and field-effect transistors on a semi-insulating substrate. The present invention fulfills this need.